Recirculation of glycol is often used in a context of transportation of a mixture of hydrocarbons comprising natural gas, water, and dissolved salts in production pipelines from an off-shore located (mineral) hydrocarbon reservoir to a land based or floating top-side facilities for processing the mixture to recover the desired hydrocarbon products. Due to shifting physical conditions during the pipeline transit, there is a problem with formation of hydrates in the fluid mixture of the pipelines threatening to clog the lines. One much applied solution to the problem of hydrate formation is to add, at subsea level, relatively low water content glycol (referred to as lean glycol) into the process fluid which usually is a mixture of hydrocarbons comprising natural gas, water, and dissolved salts, and then extract the glycol as so-called rich glycol from the process fluid at the top-side facilities. The rich glycol has a higher water content than the lean glycol. The glycol often selected for this purpose is mono ethylene glycol (MEG), in which case the lean glycol is referred to as lean MEG and the rich glycol is referred to as rich MEG.
From an operational costs and environmental point of view, the glycol stream should be recirculated. To this end, rich MEG is typically regenerated to form lean MEG and then reused as hydrate inhibiting agent in the production lines. Rich MEG usually contains remains of the hydrocarbons, high water levels, corrosion products, production chemicals and other contaminants originating from the hydrocarbon reservoir.
Typical other contaminants originating from the hydrocarbon reservoir include salts and mercury. Both may be present in the rich MEG stream. The presence of glycol is thought to increase the solubility of elemental mercury in an aqueous phase containing the glycol.
A typical MEG loop is described in Paper No. 10131 of NACE International Corrosion Conference and Expo 2010 titled “Development of a simulator for ethylene glycol loops based on solution thermodynamics and particle formation kinetics”, by Marion Seiersten et al., wherein a process stream containing natural gas is conveyed through a pipeline. A MEG stream is injected into a pipeline at a subsea injection point, and mixed with a glycol-containing stream with a production stream in the pipeline. Both are conveyed through the pipeline to receiving facilities, where MEG, formation water and condensed water are separated from the gas and condensate. The rich MEG is heated and depressurized to remove dissolved hydrocarbons. From there the rich MEG is passed via a pre-treatment to a rich MEG storage. A MEG re-concentration system having a boiler to boil off water, and a flash separator for removing salts, and a column for MEG water separation is used to recover the MEG. The recovered MEG is piped back to the subsea injection point via an injection pump after having been stored in a lean MEG storage tank. Water streams are discharged from the boiler and the column.
A concern associated with the typical MEG loop as described above is that the discharged water stream(s) may be contaminated with mercury so much that legislative requirements may preclude discharging of the water into the environment.